[Proj] Problem understanding the new API

Kristian Evers kreve at sdfe.dk
Tue Feb 20 01:44:27 EST 2018

> On 20 Feb 2018, at 06:21, Idan Miara <idan at miara.com> wrote:
> Hi,
> Which CRS is exactly the hub in the old API? Is it WGS84 lat/lon or geocentric?

WGS84 lat/lon, although if necessary the coordinates are passed through a geocentric WGS84 (when using +towgs84).

> Should it be more accurate or faster to use the new API to transform WGS84 lat/lon to WGS84 UTM,  will it use any different transformation path?

Well… It can be more accurate if you know which WGS84 realisation you are working in,  you have timestamps for your data AND you define your own custom transformation. Otherwise the accuracy will be the same. If you don’t know anything about your data then assume that the uncertainty of the transformation is 2m. If it is faster will depend on what you are doing. Initial testing show that the old and new APIs perform more or less the same for similar transformations. In general transformations will probably be slightly slower since more complicated transformations can be done now.

You CAN use a different transformation path if you want - and know how to - otherwise no, the transformation path will be the same for now, for example if you are using the epss-init file with the proj_create_crs_to_crs function. At the moment using a different transformation path requires you to set up a transformation pipeline that does what you want. In the future we will improve the situation such that at least the epsg transformations uses the optimal transformation path in a given situation.

> Thanks. 

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